1. Field of the Invention
The present invention relates to a method for producing a metal powder suitable for electronic applications, and more particularly, to a method for producing a metal powder with a fine, uniform particle size and a high degree of crystallinity which is useful as a conductive powder for use in a conductive paste, a metal powder produced by the method, a conductive paste, and a multilayer ceramic electronic component.
2. Description of the Related Art
In conductive metal powders used in conductive pastes that are used to form electronic circuits, it is desirable that these powders contain few impurities, that the powders be fine powders with a mean particle size as fine as 0.01 μm to 10 μm, that the particle size and particle shape be uniform, and that the powders have good dispersibility with no aggregation. Furthermore, it is also necessary that the dispersibility of the powder in the paste be good, and that the crystallinity be good so that there is no non-uniform sintering.
Especially in cases where such powders are used to form internal conductors or external conductors in multilayer ceramic electronic components such as multilayer capacitors, multilayer inductors and the like, in addition to being finer and having uniform particle size and shape in order to form the electrodes into a thin film, conductive metal powders are required to be resistant to the occurrence of expansion and shrinkage caused by oxidation and reduction during firing and have a high sintering starting temperature in order to prevent structural defects such as delamination or cracking. Consequently, there is a need for sub-micron sized metal powder having a spherical shape, low activity and high crystallinity.
Examples of conventional methods used to produce such a highly-crystallized metal powder include chemical vapor deposition (CVD), in which a vapor of a metal compound such as nickel chloride is reduced with a reducing gas at a high temperature, physical vapor deposition (PVD), in which a metal vapor is condensed in a gas phase, and spray pyrolysis, in which a solution or suspension of a metal compound dissolved or dispersed in water or an organic solvent is formed into fine liquid droplets, followed by carrying out pyrolysis by heating the droplets preferably at a temperature near or not lower than the melting point of the metal, whereby metal powder is produced.
In addition, methods for producing a highly-crystallized metal powder are also known in which pyrolysis is carried out at a high temperature using a solid powder for the raw material and in a state in which the solid powder is dispersed in a gas phase (see Japanese Patent Publication Nos. 2002-20809 and 2004-99992). In these methods, a highly-crystallized metal powder is obtained by supplying a raw material powder composed of a thermally decomposable metal compound powder to a reaction vessel using a carrier gas, and heating the material powder at a temperature higher than the decomposition temperature thereof and not lower than (Tm−200)° C. where Tm is the melting point (° C.) of the metal, in a state in which the material powder is dispersed in the gas phase at a concentration of 10 g/liter or less. Also, in these methods, a highly-crystallized metal powder is obtained by ejecting the raw material powder along with the carrier gas into the reaction vessel through a nozzle under the condition of V/S>600, with V representing the flow rate per unit time of the carrier gas (L/min), and S representing the cross-sectional area of the opening part of the nozzle (cm2).
In the methods described in Japanese Patent Publication Nos. 2002-20809 and 2004-99992, since in comparison with spray pyrolysis there is no energy loss resulting from the evaporation of a solvent and the metal compound powder can be dispersed in a gas phase at a high concentration due to the use of a solid metal compound powder as the starting material, a spherical metal powder having a high crystallinity and superior oxidation resistance and dispersibility can be produced with a high efficiency. In addition, a metal powder of an arbitrary mean particle size and uniform particle size can be obtained by controlling the particle size and dispersion conditions of the raw material powder, and since an oxidative gas is not generated from a solvent, these methods are also suitable for producing easily oxidizable base metal powders required to be synthesized under a low oxygen partial pressure. Moreover, in comparison with vapor phase chemical reduction methods and the like, for which it is difficult to produce an alloy of metals having different vapor pressures in an accurately controlled composition, these methods also offer the advantage of being able to easily produce an alloy powder of an arbitrary composition by using a mixture or composite of two or more types of metal compounds.
In the case of the method described in Japanese Patent Publication No. 2004-99992 in particular, a solid raw material powder is ejected into a reaction vessel through a nozzle together with a carrier gas at a high linear velocity such that V/S >600, and, utilizing the rapid expansion of the gas in the reaction vessel, it is subjected to heat treatment at a high temperature and a low concentration in a gas phase and in a highly dispersed state so as not to cause mutual collisions between raw material particles and formed particles, thereby making it possible to easily produce a metal powder having an extremely narrow particle size distribution at a low cost and high efficiency.
There has recently been a strong demand for multilayer ceramic electronic components having a reduced size and increased layering, and in the area of multilayer ceramic capacitors using nickel for the internal electrodes in particular, both the ceramic layer and internal electrode layer are becoming increasingly thin. Consequently, an ultrafine nickel powder is required for use in the conductive paste for these internal electrodes that has, for example, an extremely small mean particle diameter of 0.3 um or less, a minimized inclusion of coarse particles and a narrower particle size distribution.
However, in the case of attempting to produce an even finer nickel powder than in the past using the methods described in Japanese Patent Publication Nos. 2002-20809 and 2004-99992, there are problems with respect to a tendency for the particle size distribution to increase along with poor production efficiency and yield.
These problems are presumed to be attributable to the following causes. Namely, in the methods described in Japanese Patent Publication Nos. 2002-20809 and 2004-99992, since nearly one metal particle or alloy particle is formed per particle of raw material, powder, the particle size of the metal powder is dependent on the particle size of the raw material powder. Thus, in order to obtain a finer metal powder, it is necessary to finely pulverize and disaggregate the raw material powder in advance. However, since the cohesive force of a powder increases as it becomes finer, it becomes difficult to disperse in which in addition to requiring an extremely long time for the disaggregation step, requiring a large amount of energy and contributing to poor production efficiency, large particles are formed easily due to re-aggregation. If such large, aggregated particles that are unable to be completely disaggregated are present in the raw material powder in this manner, the particle size and particle size distribution of the resulting metal particles increase. In addition, due to the inclusion of coarse metal particles, there are various detrimental effects on the characteristics of multilayer ceramic electronic components.